In the related art, with recent progress in nano-electronics, products utilizing physical phenomena inherent in minute-sized magnetic materials have been developed. Particularly developments are rapid in the technical fields using the spin of free electrons in the magnetic materials (hereinafter, referred to as “spin electronics”).
In the field of spin electronics, the most realistic application is a spin valve element utilizing a tunneling magnetoresistance (TMR) effect which occurs in a multi-layered structure including a ferromagnetic layer/an insulating layer/a ferromagnetic layer and a giant magnetoresistance (GMR) effect which occurs in a multi-layered structure including a ferromagnetic layer/a non-magnetic layer (conductive layer)/a ferromagnetic layer. Herein, the symbol of slash “/” written between layers represents that the layers before and after the symbol are stacked in order.
FIGS. 1 and 2 illustrate examples of the structure of a conventional spin valve element. FIG. 1 illustrates a basic configuration of a spin valve element utilizing the TMR effect. The spin valve element is configured to include an insulating layer 30, an insulating layer 24, a pair of ferromagnetic layers (a pinned layer 23 and a free layer 25) that sandwiches the insulating layer 24, and electrode layers 21 and 27, in which all the above layers are formed on a substrate 5. If necessary, an anti-ferromagnetic layer (pinning layer) 22, a capping layer 26, or the like may be further included. Magnetization of the pinned layer 23 is pinned by magnetic coupling with the anti-ferromagnetic layer 22. Magnetization of the free layer 25 is controlled by the external magnetic field or the spin injection originating from a spin-polarized current.
In the control using the spin injection, if electrons are caused to flow to an element from the pinned layer 23, a spin torque is exerted on the spin of the free layer 25 such that the spin of the free layer 25 becomes parallel to that of the pinned layer 23. On the other hand, if electrons are caused to flow from the free layer 25 to the pinned layer 23, a spin torque is exerted on the spin of the free layer 25 such that the spin of the free layer becomes anti-parallel to that of the pinned layer 23. Because of this action, the direction of the magnetization of the free layer 25 can be controlled by the direction of a current (magnetization reversal by spin injection).
In this manner, the direction of magnetization of the free layer is rotated, or reversed according to external magnetic field or spin injection originating from the spin-polarized current. Therefore, the resistance of the element greatly changes depending on the direction of the magnetization of the free layer with respect to that of the pinned layer. In other words, the element is configured to have the highest resistance value when the magnetization vectors of the two layers are in anti-parallel to each other and the lowest resistance value when the magnetization vectors of the two layers are in parallel to each other.
FIG. 2 illustrates a basic configuration of a spin valve element using the GMR effect, which is different from the spin valve element using the TMR effect illustrated in FIG. 1 in that the insulating layer 24 is substituted with a non-magnetic (conductive) layer 51, but other functions thereof are basically the same.
There has been proposed a solid state magnetic memory using such elements to record information, one bit per element. In addition, there has also been proposed a multi-value recording technology capable of recording information, two bits per element. In addition, since two states (two values) can be recorded for one bit, a total of four states (four values) can be recorded with 2 bits.
However, since the conventional multi-value recording technology still involves several problems, it has not yet come to the practicable stage. For example, JP-A No. 10-91925 discloses a solid state magnetic memory based on a multi-value recording technology capable of recording information, two bits per element, with use of a double-tunnel junction element having a multi-layered structure including a ferromagnetic layer/a first insulating layer (or a first non-magnetic layer)/a ferromagnetic layer/a second insulating layer (or second non-magnetic layer)/a ferromagnetic layer. In this solid state magnetic memory, the element needs to have multiple structures. Therefore, between one multi-layered structure including a ferromagnetic layer/a first non-magnetic layer/a ferromagnetic layer and the other multi-layered structure including a ferromagnetic layer/a second non-magnetic layer/a ferromagnetic layer, the output voltage level measured between the ferromagnetic layers in each structure is distinguishable. For this reason, in the solid state magnetic memory disclosed in JP-A No. 10-91925, if a magnetoresistance ratio of at least one of the two multi-layered structures included in the solid state magnetic memory is configured not to be larger than a magnetoresistance ratio of a TMR element having a single-tunnel junction of a structure including a ferromagnetic layer/a non-magnetic layer/a ferromagnetic layer (the other TMR type element), there is a problem in that a sufficient S/N ratio cannot be obtained.
JP-A No. 2003-31771 discloses a method of recording information, using a structure in which two ferromagnetic layers are multi-layered with a non-magnetic layer interposed therebetween in a manner such that the directions of the magnetizations of the ferromagnetic layers are perpendicular to each other. With such a combination, each of the ferromagnetic layers individually stores one bit, that is, a total of 2 bits. In other words, four states are recorded in the two ferromagnetic layers. However, this method is disadvantageous in that it requires multiple structures and it needs to perform a switching operation with use of the external magnetic field by generating two directional magnetic fields (forward and backward) for each ferromagnetic layer, that is, generating a total of four directional magnetic fields.
JP-A No. 2007-317895 discloses a structure in which two standby portions are disposed adjacent to a free layer and notches for pining magnetic walls are disposed so as to correspond to the standby portions. However, this technique has a problem with an increased area in the lateral direction due to the area occupied by the standby portions. For such a reason, although the multi-value recording is achieved, the improvement of the recording density is unsuccessful.
Japanese Patent Application National Publication (Laid-Open) No. 2005-535111 discloses a free layer that has plural stabilized sites thanks to the shape anisotropy of the free layer. However, this technique has a problem in that achievement of the shape anisotropy leads to warping of a shape (distortion) and an increase in the reversal magnetic field.
In addition, M. Rahm et al., “Influence of Point Defects on Magnetic Vortex Structures,” Journal of Applied Physics, Vol. 95, 6708, American Institute of Physics, Jun. 1, 2004, discloses a configuration in which “defects” are disposed linearly on a circular magnetic film and vortexes of the magnetization are caused to be positioned at the defects.
In the configuration, determining at which defects the vortexes are to be positioned is controlled by the external magnetic field to implement plural magnetic states. However, according to this disclosure, since the arrangement of the “defects” serving as recoding points is limited to the linear fashion, practically it is difficult to increase the density of the recording points. In addition, Rahm et al. does not disclose a configuration of a spin valve element having a simple reading function.
In view of the above-mentioned problems, an object of the invention is to provide a multi-value recording spin valve element capable of allowing achievement of a high recording density without using a complicated structure such as multiple structures, a method of driving the spin valve element, and a storage device using the spin valve element.